Don't leave path behind when executing :chflags_success

Prior to this change a SF_IMMUTABLE chflagsat(2)'ed file (path) was left
behind, which sabotaged kyua(1) from being able to clean up the work directory,
This resulted in unnecessary work for folks having to clean up the work
directory on non-disposable systems, which defaults to /tmp. Use UF_OFFLINE
instead of SF_IMMUTABLE, in part because setting SF_IMMUTABLE isn't relevant
to the test and SF_IMMUTABLE cannot be cleared at all securelevels, as pointed
out by @asomers.

Additional work is required to catch cases like this upfront in the future to
avoid tester headache. See PR # 247765 for more details/followup.

Suggested by: asomers
Reviewed By: asomers, tests
MFC after: 1 week
PR: 247761
Sponsored by: DellEMC
Differential Revision: https://reviews.freebsd.org/D25561